1. Field of the Invention
The present invention relates to a spring-actuated electromagnetic brake having a noise suppressing device for suppressing a striking noise generated when an armature is attracted onto a field upon magnetization of the field to release the brake.
2. Description of the Related Art
Noise suppressing devices incorporated in conventional spring-actuated electromagnetic brakes so as to reduce a striking noise emitted when an armature is attracted onto a magnetized field to release the brake are generally classified into five groups as enumerated below.
(1) An O-ring fitted in an annular groove or recess formed in one of two opposed surfaces of the field and armature in such a manner that the O-ring partially projects from the annular recess when the brake is in the engaged state (Japanese Patent No. 2669944).
(2) A cushioning sheet member disposed between the field and the armature (Japanese Utility Model Laid-open Publication No. HEI 5-25994).
(3) A plurality of tubular noise-suppressing rubber blocks each received in one of plural recesses formed in an attracting surface of the field in such a manner that when the brake in the engaged state, the tubular noise-suppressing rubber block partially projects from the attracting surface (Japanese Utility Model Laid-open Publication No. HEI 7-16576).
(4) A plurality of tubular noise-suppressing rubber blocks each received in one of plural recesses formed in an attracting surface of the field in such a manner that the tubular noise-suppressing rubber block partially projects from the attracting surface when the brake is in the engaged state. In combination with the tubular noise-suppressing rubber blocks, an armature formed from a vibration-damping steel sheet is used (Japanese Utility Model Laid-open Publication No. HEI 8-7808).
(5) A plurality of solid cylindrical noise-suppressing rubber blocks each received in one of plural recesses formed in an attracting surface of the field in such a manner that the solid cylindrical noise-suppressing rubber block partially projects from the attracting surface when the brake is in the engaged state.
The conventional noise-suppressing devices enumerated above still each have a problem, as discussed below.
(1) The O-ring, if the diameter of its circular cross section is small, can achieve only a limited elastic deformation, failing to perform the desired noise suppressing function. By contrast, if the diameter of the circular cross section of the O-ring becomes large, a correspondingly large cross-sectional area is required for the annular recess, which will deteriorate attracting power of the field.
(2) The cushioning sheet member disposed between the field and the armature exerts direct influence on the attracting power of the field. Namely, the larger the thickness of the cushioning sheet member, the lesser the attracting power of the field. By contrast, as the thickness of the cushioning sheet becomes small, the durability of the cushioning sheet member decreases.
(3, 4 and 5) The hollow or solid cylindrical noise-suppressing rubber blocks incorporated in the spring-actuated electromagnetic brake are shown in FIGS. 6A and 7A. The brake shown in FIG. 6A is in its engaged position, while the grake shown in FIG. 7A is in its released position. When the brake shown in FIG. 6A is to be released, an electromagnetic coil 2 embedded in a field 1 is excited to magnetize the field. Upon magnetization of the field, an armature 3 is attracted onto the field 1. As the armature 3 moves toward the field, the cylindrical noise suppressing blocks 11' (FIG. 7B) are axially compressed by the armature 3. In this instance, due to the cylindrical shape of the rubber blocks 11', it is likely that the cylindrical rubber blocks undergo local elastic deformation concentrated on respective tip end portions, as best shown in FIG. 7B. The locally concentrated elastic deformation will soon deteriorate the durability of the rubber blocks.
(4) The armature formed from a vibration-damping steel sheet required an increased number of parts used and hence is relatively expensive to manufacture. Additionally, in order to generate the necessary attraction, the armature has a relatively large thickness. This will add to the overall size of the brake.